This invention relates generally to densitometers for measuring optical density. In particular, the invention relates to optical density measurement of multiple spots or test patches for monitoring and quality control of printed output.
In printing and copying apparatus, machine parameters are adjusted, either manually or automatically, to produce images having well regulated darkness or optical density. Printer process control strategies typically involve automatically measuring the transmissive or reflective optical density of printed areas (called xe2x80x9ctest patchesxe2x80x9d) as they are printed. Alternatively, printed samples may be measured manually using a xe2x80x9cbench-topxe2x80x9d densitometer. In either case, the density measurements are the basis for quality evaluation and control. Adjustments may then be determined to regulate the printed test patches to the desired density levels. The adjustments are often applied automatically, though some printer adjustments may require manual adjustment by an operator. For test and diagnostic purposes, large nominally uniform areas or multiple test patches may be printed to check for deviations from the desired uniformity.
In a large-format printer, uniformity may be difficult to maintain over the large printed areas, particularly in the direction perpendicular (cross-track) to the process direction. Nonuniformity can also be a problem in non-printed web production processes, such as plastic sheet, textiles, and paper. In these cases optical density measurements may be needed transversing the cross-track direction. U.S. Pat. No. 5,546,165, to Rushing et al uses a document scanner as a test print densitometer. Such a scanner may use an essentially continuous linear array of light sensors to collect measurements from several thousand picture elements or pixels spanning the cross-track dimension. These measurements provide the cross-track density xe2x80x9cprofilexe2x80x9d. This approach requires uniform illumination across the wide printed area, imaging optics, and a shift register driven to provide an output voltage signal representing the pixel-by-pixel cross-track profile of transmittance or reflectance.
A simpler and less expensive approach is to obtain representative cross-track measurements at just a few spots. Three or four representative cross-track measurements are sufficient to guide basic adjustments and/or maintenance on the typical machine. The objective of such basic adjustments and/or maintenance is typically to balance the average density edge-to-edge, and avoid a xe2x80x9chighxe2x80x9d or xe2x80x9clowxe2x80x9d spot in the center.
Adjustments to the engagement or spacing of the various work stations, relative to the image-bearing medium, are often used. Independent adjustments are often available at the front and back ends of a work station. An independent center adjustment may also be available. In an electrophotographic process, such adjustments may be applied to corona charging devices, to illumination or exposure, and to toning stations. Image exposure adjustments, gradual from edge-to-center-to-edge, are disclosed in U.S. Pat. No. 5,933,682 to Rushing, for example. In paper manufacturing processes discussed by Gorinevsky et al, sets of identical independent actuators, distributed across the paper web, control cross-track uniformity of web attributes at several production stages.
The electrophotographic printer described in U.S. Pat. No. 5,983,044 to Kodama et al has a densitometers positioned before and after transfer of the toned image from the photoconductor drum to the receiver. One densitometer is positioned to read test patches on the drum before transfer. Two more densitometers provide post-transfer readings of the transferred toner patch and the residual toner remaining on the photoconductor drum, respectively. Transfer efficiency is determined from these readings. Deviations from the normal transfer efficiency are the basis for electrical adjustments applied to the multi-color developing units and to the transfer process.
In a typical multi-color printer, test patches of each process color are monitored for process control purposes. Ideally, each color has its own dedicated densitometer channel, with a light emitter color or peak emission wavelength selected for high sensitivity of the readings. Separate dedicated channels may also enable density readings to be taken farther upstream in the imaging process, in the individual color processing modules, before the separations are collected on a single web. The earlier upstream readings minimize the delay in obtaining measurements, and enable faster-responding feedback control loops for the process.
An economical single-channel densitometer for an electrophotographic color printer is disclosed in U.S. Pat. No. 5,075,725 to Rushing et al. The transmission densitometer has an infrared emitter and measures test patches covered with cyan, magenta, or yellow toner. Previously stored base density readings from untoned film are subtracted to yield net toner densities. The machine logic keeps track of the color of the passing patches, so that the measured net toner densities can be compared to target values for the respective colors.
This single-channel infrared-emitting densitometer outputs usable density signals for patches covered with the colored toners, and black toner. However, colorants in other applications, such as ink jet inks, do not sufficiently block infrared light. Even for unfused colored toners, better sensitivity is obtained using emitters of complementary color to the respective test patches, i.e., red, green, and blue emitters for cyan, magenta, and yellow toner, respectively. Finally, the single-channel configuration does not address the need in some printer configurations for density readings at multiple positions.
The approach described in U.S. Pat. No. 3,995,958 to Pfahl et al obtains good color sensitivity. Filters of complementary colors to the test patches automatically rotate into position in front of a white light emitter, so that each colored patch is read with light of the particular color for highest sensitivity. While using only one densitometer, this approach requires bulky, complicated, and expensive mechanisms to change filter positions at the appropriate times. Furthermore, all the variously colored patches must be on the same single track of the moving medium, and pass the densitometer one after another. Such a fixed-position single-channel configuration cannot address needs for density readings in multiple in-track or cross-track positions.
As an alternative to the white light source and multiple color filters used in U.S. Pat. No. 3,995,958 by Pfahl et al, multiple emitters of different colors can be used, e.g., red, green, and blue LEDs. These emitters are aimed such that transmitted or reflected light is collected by a single photodetector. To read cyan, magenta, and yellow test patches, a single complementary-colored LED is energized, according to the known color of the test patch to be read at that time. A test patch of mixed or layered colorants is read by rapidly and successively energizing the LEDs one at a time, to obtain a set of density readings characteristic of the overall test patch color while the test patch is in position. Such a configuration measures test patches of various colors without the complexity of mechanical motion in the densitometer, but otherwise is subject to the same limitations as other fixed-position single-channel configurations.
Using mechanical drives, a single-channel densitometer configuration can be adapted to obtain readings at different positions. In U.S. Pat. No. 4,003,660, Christie et al disclose a densitometer movably mounted on support members that extend across the width of a printed web. Such a mechanical drive is bulky, complicated, and expensive. Optical spacing and alignment tolerances are-more difficult to maintain, owing to the motion. At any given time, readings can be obtained at only one position, and time is required to move to the next position.
Multiple densitometers have been incorporated into computer networks, as in U.S. Pat. No. 5,402,361 to Peterson et al. Such networks facilitate consolidation of data into a central collection point, or host computer, for data logging, printing, evaluation, and process control purposes. The host computer in such a network processes signals for the individual connected densitometers, and can also compute multi-channel functions requiring data from two or more densitometers. A simple example of a multi-channel function is the density difference between two densitometers. However, such networks alone do not reduce the cost of the densitometers themselves.
Many document reproduction centers, printing shops, and graphic arts work areas have multiple printers, with many of the printers requiring at least one xe2x80x9con-boardxe2x80x9d densitometer. In both on-board and bench-top applications, multi-channel densitometry facilitates uniformity evaluation and multi-color test patch measurements. However, the cost of multiple densitometers, or multi-channel densitometry, is considerable and often prohibitive.
High cost (typically $1000 or more each) has not been a serious deterrent to the use of bench-top densitometers in professional laboratories. However, in more cost-sensitive environments, such as amateur photography labs, graphic arts studios, and student laboratories at educational institutions, bench-top densitometer cost has been an obstacle. On-board densitometers, specialized for use in a specific machine, and with minimal or no operator interfaces, can be much less costly than general-purpose bench-top instruments. Nevertheless, cost is an obstacle to their wide use within moderately priced copiers, printers, and other products. Cost issues are multiplied, of course, when multichannel densitometry is needed.
Advancing component technologies are reducing the aforementioned cost barriers, making densitometers cost-effective in an increasing range of applications. On-board densitometers have gradually penetrated the printer market, down even to some moderately priced printers. Less costly and more reliable LEDs are now often used instead of incandescent lamps as densitometer light emitters. Photodiode light detectors are smaller and less costly than the photomultiplier tubes used in some older dk densitometers. Application of digital electronics to densitometers eliminates the need for the costly analog logarithmic amplifier used in traditional analog designs. For example, U.S. patent application Ser. No. 10/095,166 of Rushing discloses an all-digital approach based on a light-to-frequency (L-to-F) converter electrically interfaced to a microcontroller and utilizing a look-up table (LUT). Despite these cost-reducing advances, densitometer cost is still an issue in printer design, particularly when multiple densitometers are considered for moderately priced products. The cost of bench-top densitometry, particularly multi-channel densitometry, also remains an issue in amateur photo labs, student laboratories, and other cost-sensitive areas.
One object of the present invention is to reduce densitometer cost for density measurement at multiple positions, for both bench-top and on-board applications. A single controller circuit, preferably with a digital microcontroller, provides electrical power, control signals, and sensor signal processing for multiple channels with preferably digital light detectors in one or more probes. With the preferable digital light detectors, the costly analog logarithmic amplifiers of traditional analog densitometers are eliminated. The controller circuit may be located on a probe along with the sensors for one or more channels, or may be on a separate connected circuit board. In either case, the components of the controller circuit are not replicated for each channel, further cutting costs.
Typically, the densitometer measurement channels are located on a single machine, or at a single bench-top work area. However, probes in multiple machines may also be connected to a single controller circuit, especially if the machines are close to each other and operated together.
Each densitometer probe contains at least one light sensor, in which the light-detecting component is preferably a small L-to-F converter integrated circuit. The corresponding light emitters, preferably LEDs, may also be included on the probe. The LEDs, if not a part of the probe, are separately mounted in positions that, align with the sensors on the probe when the probe is in its operating position. Separate LED mounting is sometimes preferable in a transmission mode of operation, where the LED and light detector are on opposite sides of the sample.
For mounting the probes, slide rails facilitate easy installation and removal, and establish a well-defined position relative to the sample to be measured. Alternatively, a mounting block at the connector end of the probe facilitates attachment to a support structure for cantilever mounting. The connector at the end of the probe is disconnected for easy probe removal, such as for cleaning. Should a probe become damaged or inoperative, only that one probe need be replacedxe2x80x94not the other probes or the separate controller circuit board.
Another object of the invention is a reduced space requirement at the measurement locations. By limiting the probe components and function to the minimum required to output digital signals responsive to light impinging on the light detectors, the probes can be made small. In particular, the probe width, in the process direction, can be minimized. An L-to-F converter in integrated circuit form, along with a controller circuit serving multiple channels, minimizes the total component count With a separate controller circuit board, the probe electronic components for each channel may consist of only the L-to-F converter, a decoupling capacitor, and the LED emitter with a series resistor (unless the LED is separately mounted). In some applications the probe may include additional sensor components such as light shields, lenses, and color filters.
With less space required by the densitometer probes, more space is available for various work stations, or for enabling overall reduction in machine size. Alternatively, the small probe may permit a needed density measurement where it could not be done with a bulkier complete densitometer. The small probe size, along with the electrical connector and mounting provisions, facilitate probe removal and replacement. Relocation of a probe to another measurement position is also easier, if that should be necessary.
Yet another object of the invention is to compute multi-channel, as well as single-channel, density functions in the densitometer controller circuit, where the density signals from all the channels are collected. Evaluations of uniformity or transfer efficiency, for example, require multi-channel density measurements. Only the required calculated results, measurement summaries, statistics, or exceptions are sent to the host computer or display device. This unburdens the host computer from such computations, allowing it to better and more timely attend to higher-level machine control functions. If the connection to-the host is wireless, outputting only summary data may also be advantageous in terms of reduced time duration of the transmission and/or reduced bandwidth.
Still another object of the invention is superior noise immunity, obtained by utilizing an all-digital approach. The issue of electrical noise immunity is heightened in multi-channel and multi-probe configurations, owing to the multiplicity of interconnections in the generally noisy environments inside printers or other machines. Electrophotographic printers, for example, contain noisy devices such as motors and corona chargers, mating noise immunity essential for accurate multi-channel densitometry. In some prior art densitometers, analog signals are switched, which can introduce additional transient noise or steady error. In the present invention, the preferred L-to-F converter integrated circuit has a digital (logic xe2x80x9chighxe2x80x9d or logic xe2x80x9clowxe2x80x9d) frequency output The digital output has inherently better noise immunity than the analog input and output signals associated with the photodiodes, linear amplifiers, logarithmic amplifiers, and analog-to-digital converters of traditional densitometer designs. There is no switching of sensitive analog signals.
To obtain these objects, the multi-channel densitometer in the preferred embodiments utilizes an all-digital design and a single controller circuit to control and process signals to and from multiple measurement channels. The microcontroller of the controller circuit controls and processes signals individually for the channels, and also computes multichannel functions requiring readings from two or more channels. The L-to-F converter integrated circuits, one for each channel, provide digital frequency outputs. Control signals are also exclusively digital. This gives the multi-channel densitometer the superior noise immunity inherent in digital signals. The L-to-F converter integrated circuits also contribute to the minimal component count, small probe size, and economical cost.